This invention relates to the testing of electrical apparatus for corona faults, and more particularly to an improved method and system for locating corona sources in transformers and other electrical apparatus by sensing and averaging the acoustic noise generated by a corona discharge.
The site of incipient corona faults in encased electrical apparatus such as large transformers can be located by determining the time for sound waves produced by a corona discharge to propagate to an acoustic sensor at a known position. As a timing reference to indicate initiation of the corona discharge, the high frequency corona electrical signal superimposed on the power frequency output voltage is detected. By measuring the acoustic propagation time from a corona source to several sensor positions, and with knowledge of the acoustic velocity in the liquid or gas filling the transformer housing, the distances to the corona source can be calculated and the location determined by triangulation. Repair efforts to replace or repair the faulty section may then proceed more efficiently because of the reduced amount of disassembly that is needed.
The accurate detection of an acoustic signal representing the received sound waves is made difficult because of the noisy acoustic environment within the transformer housing. Often there are several corona discharges from the same source within a half cycle of the excitation frequency, and the acoustic noise from each discharge reverberates within the housing for a significant amount of time. There is also background noise, and other noise sources in the transformer which resonate at the frequency of the discharge noise are excited and contribute to the noise. For this reason, averaging or integrating the acoustic signal over a large number of cycles has been employed to improve the signal-to-noise ratio, and is further described for instance in U.S. Pat. No. 3,430,136 to H. F. Brustle et al. In this patent, a large corona electrical pulse is used as a timing reference to start the signal averaging in each half cycle, reasoning that a large corona pulse should produce a large acoustic signal resulting in a good signal-to-noise ratio. This is true many times, but when the large corona pulse is preceded by other pulses of almost the same energy the resultant reverberations produce a poor signal-to-noise ratio. Since the test time is limited, signal averaging over the available number of half cycles will not sufficiently enhance the signal-to-noise ratio to allow the operator to locate the fault.
Other prior art approaches, not necessarily relying on acoustic signal averaging, initiate acoustic signal testing at only the peaks of the transformer output voltage, or during a predetermined fixed portion (or fixed window) of the transformer output voltage half cycle. All of these schemes have their merits, but none is successful in locating corona faults when the corona discharges are so close together that large reverberations swamp the acoustic signal. Normally, this occurs to a moderate extent in all cases where corona exists. Furthermore, multiple corona sources confused the operator.